1. Field of the Invention
The present invention relates to an information recording medium like an optical disc which has large capacity and to/from which data are written/read in high speed, and more particularly to a recording medium which can perform after-recording, an apparatus and a method for recording thereto.
2. Related Art
In the field of a writable optical disc having an upper bound of approximately 650 MB, a phase change type disc DVD-RAM having a capacity of several GB has appeared. Moreover, in addition to the practical use of MPEG (MPEG2) which is the coding standard of digital AV data, the DVD-RAM has been expected as recording and reproducing media in the AV field as well as computer application. In other words, it is expected that the DVD-RAM will spread as media in place of a magnetic tape which is conventionally typical AV recording media.
(Description of DVD-RAM)
In recent years, an enhancement in the density of a writable optical disc has been developed so that it has been possible to record video data as well as computer data and audio data.
For example, a convexo-concavo shaped guide groove has conventionally been formed on the signal recording face of the optical disc. While a signal has conventionally been recorded on only a land portion or a groove portion, it has become possible to record the signal on both the land and groove portions by a land-groove recording method. Consequently, a recording density has been enhanced by approximately twice as much (see Japanese Patent Laid-Open Publication No. 8-7282, for example). Moreover, there has also been devised and practically used a zone CLV method or the like in which the control of a CLV method (constant linear velocity recording) effective in an enhancement in the recording density can be simplified and easily used practically (see Japanese Patent Laid-Open Publication No. 7-93873, for example).
Significant problems in the future are how to record AV data including video data using an optical disc intended to have an increase in a capacity, and how to implement performance greatly exceeding a conventional AV apparatus and new functions.
By the appearance of such a writable optical disc having a large capacity, it can be supposed that an optical disc becomes a mainstream also for AV recording and reproduction in place of a conventional tape. The conversion of recording media from the tape into the disc has various influences on the function and performance of an AV apparatus. The conversion into the disc has the greatest feature that random access performance is considerably enhanced. If the tape is subjected to random access, it is necessary to usually take a time in order of several minutes for one rewinding. This is extraordinarily late as compared with a seek time (20-60 ms or less) on the optical disc media. Accordingly, the tape cannot act as a random access device in respect of practical use. By such random access performance, the distributed recording operation of the AV data which could not be performed by the conventional tape can be implemented by the optical disc.
FIG. 34 is a block diagram showing a drive device of a DVD recorder. In the drawing, the reference numeral 11 denotes a optical pick-up for reading the data of a disc, the reference numeral 12 denotes an ECC (error correcting code) processing section, the reference numeral 13 denotes a track buffer, the reference numeral 14 denotes a switch for switching the input and output to and from the track buffer, the reference numeral 15 denotes an encoder, and the reference numeral 16 denotes a decoder. The reference numeral 17 denotes a enlarged part of the disc.
As shown by the reference numeral 17, data are recorded on the DVD-RAM disc with 1 sector=2KB as a minimum unit. Moreover, an error correcting processing is executed by the ECC processing section 12 with 16 sectors=1 ECC block.
The track buffer shown by the reference numeral 13 serves to record the AV data with a variable bit rate in order to record the AV data on the DVD-RAM disc more efficiently. While a read/write rate (Va in the drawing) from/to the DVD-RAM is a fixed rate, the AV data change a bit rate (Vb in the drawing) according to the complexity of the contents thereof (a video image, for example). The track buffer 13 serves to absorb a difference in the bit rate. For example, this is not required if the AV data is set to the fixed bit rate as in a video CD.
By utilizing the track buffer 13 still effectively, the AV data can be discretely provided on the disc. Description will be given with reference to FIG. 35.
FIG. 35A is a diagram showing an address space on a disc. In the case where the AV data are separately recorded in a continuous region of [a1, a2 ] and a continuous region of [a3, a4 ] as shown in FIG. 35A, the AV data can be continuously reproduced by supplying data stored in the track buffer 13 to the decoder 16 while a seek is being carried out from a2 to a3. A status obtained at this time is shown in FIG. 35B.
The AV data read from a1 are input to the track buffer 13 and output from the track buffer 13 at a time t1, and the data are stored in the track buffer 13 by a rate difference (Va-Vb) between an input rate (Va) to the track buffer 13 and an output rate (Vb) from the track buffer 13. This state continues up to a2 (time t2). When an amount of data stored in the track buffer 13 for this period of time is represented by B (t2), it is sufficient that amount B (t2) stored in the track buffer 13 can be consumed to be continuously supplied to the decoder 16 until a time t3 corresponding to data reading start point of a3.
In other words, if a constant amount of data ([a1, a2 ]) to be read before the seek or more are kept, the AV data can be continuously supplied even if the seek is generated.
In the above example the description is given to the case where the data are read from the DVD-RAM (that is, playback), however the case where the data is written to the DVD-RAM (that is, picture recording) may be considered similarly.
If the constant amount or more of data are continuously recorded on the DVD-RAM as described above, continuous reproduction/picture recording can be carried out even if the AV data are distributed and recorded on the disc.
(Description of MPEG)
Next, description will be given to the AV data.
As described earlier, the AV data to be recorded on the DVD-RAM use an international standard referred to as MPEG (ISO/IEC13818).
Even a DVD-RAM having a large capacity of several GBs does not always have a sufficient capacity for exactly recording non-compressed digital AV data. Therefore, a method for compressing and recording the AV data is required. As a method for compressing the AV data, the MPEG (ISO/IEC13818) has widely spread in the world. In recent years, the LSI technology has been improved so that MPEG codec (expansion/compression LSI) has been put into practical use. Consequently, the DVD recorder can implement MPEG expansion/compression.
The MPEG mainly has the following two features in order to implement highly efficient data compression.
A first feature is that a compressing method using a time correlation characteristic between frames is introduced in addition to a compressing method using a space frequency characteristic which has conventionally been carried out in the compression of the motion picture data. In the MPEG, each frame (which will be also referred to as a picture in the MPEG) is classified into three kinds of parts, that is, an I picture (intra-frame coding picture), a P picture (a picture using the intra-frame coding and a reference relationship in the past) and a B picture (a picture using the intra-frame coding and reference relationships in the past and future), thereby performing data compression.
FIG. 36 is a diagram showing a relationship among the I, P and B pictures. As shown in FIG. 36, the P picture refers to the last I or P picture in the past, and the B picture refers to the closest I or P picture in the past and future. As shown in FIG. 36, moreover, since the B picture refers to the I or P picture in the future, the display order (display order) of each picture and the order (coding order) on the compressed data may be coincident with each other.
A second feature of the MPEG is that coding amount can be assigned dynamically for each picture in accordance with the complexity of the picture. The decoder of the MPEG comprises an input buffer. The decoder can assign a large coding amount to a complex picture which is hard to compress by storing data in the decoder buffer in advance.
Audio data used by the DVD-RAM can be selected for use from three kinds of parts, that is, MPEG audio for carrying out data compression, Dolby digital (AC-3) and non-compressed LPCM. While the Dolby digital and the LPCM have a fixed bit rate, the MPEG audio has a variable bit rate and has a size which is not as great as the size of a video stream but can be selected from plural kinds of sizes in an audio frame unit.
Such AV data are multiplexed into one stream by a method referred to as an MPEG system. FIG. 37 is a diagram showing the structure of the MPEG system. The reference numeral 41 denotes a pack header, the reference numeral 42 denotes a packet header, and the reference numeral 43 denotes a payload. The MPEG system has a hierarchical structure which is referred to as a pack and a packet. The packet comprises the packet header 42 and the payload 43. The AV data are divided per proper size from the head, and are stored in the payload 43. The packet header 42 stores. ID (stream ID) for identifying stored data, a decoding time DTS (Decoding Time Stamp) of data (the DTS is omitted if the decoding and the display are carried out at the same time as in the audio data) and a presentation time PTS (Presentation Time Stamp) of the data which are included in the payload represented with a precision of 90 kHz are recorded, as information related to AV data stored in the payload 43. The pack is a unit having a plurality of packets together. In case of the DVD-RAM, one pack is used every packet. Therefore, the pack comprises the pack header 41 and the packet (the packet header 42 and the payload 43). The pack header stores SCR (System Clock Reference) representing with a precision of 27 MHz a time that data in the pack are input to the decoder buffer.
In the DVD-RAM, such an MPEG system stream is recorded by using one pack as one sector (=2048 B).
Next, description will be given to a decoder for decoding the above-mentioned MPEG system stream. FIG. 38 shows a decoder model E-STD (Extended System Target Decoder) of the MPEG system decoder. The reference numeral 51 denotes an STC (System Time Clock) acting as a reference time in the decoder. The reference numeral 52 denotes a demultiplexer for decoding or demultiplexing a system stream. The reference numeral 53 denotes an input buffer of a video decoder. The reference numeral 54 denotes a video decoder. The reference numeral 55 denotes a reorder buffer for temporarily storing the I and P pictures to absorb a difference between the data order and the display order which is made between the I and P pictures and the B picture as described above. The reference numeral 56 denotes a switch for adjusting the order of the outputs of the I and P pictures stored in the reorder buffer and the B picture. The reference numeral 57 denotes an input buffer of an audio decoder. The reference numeral 58 denotes an audio decoder.
Such an MPEG system decoder serves to process the above-mentioned MPEG system stream in the following manner. At a time that the time of the STC 51 is coincident with the SCR described in the pack header, the demultiplexer 52 inputs the same pack. The demultiplexer 52 serves to interpret a stream ID in the packet header and to transfer the data of the payload to the decoder buffer for each stream. Moreover, the demultiplexer 52 fetches the PTS and the DTS in the packet header. The video decoder 54 fetches picture data from the video buffer 53 at a time that the time of the STC 51 is coincident with the DTS to carry out a decode processing, and stores the I and P pictures in the reorder buffer 55 and displays the B picture. While the I and P pictures are decoded by the video decoder 54, the switch 56 is connected to the reorder buffer 55 to output a previous I or P picture in the reorder buffer 55. While the B picture is decoded, the switch 56 is connected to the video decoder 54. The audio decoder 58 fetches and decodes data for one audio frame from the audio buffer 57 at a time that the time of the STC 51 and the PTS (there is no DTS in cast of audio) are coincident with each other in the same manner as the video decoder 54.
Next, description will be given to a method for multiplexing the MPEG system stream with reference to FIGS. 39A-39D. FIG. 39A shows a video frame, FIG. 39B shows a status in the video buffer, FIG. 39C shows the MPEG system stream, and FIG. 39D shows audio data. An axis of abscissa indicates a time base which is common to each drawing, and each drawing is represented on the same time base. As shown in FIG. 39B, moreover, an axis of ordinate indicates a buffer usage (the data storage amount of the video buffer), and a thick line in the drawing indicates a transition of the buffer usage on a time basis. Furthermore, the gradient of the thick line is equivalent to the bit rate of the video, and indicates that data are input to the buffer at a constant rate. A reduction in the buffer usage at a constant interval indicates that the data are decoded. Besides, the intersection of an oblique dotted line and the time basis indicates a time that the data transfer of the video frame to the video buffer is started.
Hereinafter, a complex image A in the video data will be described as an example. Since an image A requires a large coding amount as shown in FIG. 39B, the data transfer to the video buffer should be started at a time t1 in the drawing in place of the decoding time of the image A. (A time from the data input start time t1 till the decoding will be referred to as vbv_delay). As a result, the AV data are multiplexed in the position (time) of the video pack shown in an oblique line. On the other hand, the transfer of the audio data which does not require dynamic coding amount control differently from the video data does not need to be particularly made earlier than the decode time. For this reason, generally, the multiplexing is carried out a little before the decode time. Accordingly, the video data and the audio data which are reproduced at the same time are multiplexed in the state in which the video data are preceded. In the MPEG, a time that data can be stored in the buffer is restricted, and all the data but still picture data are defined such that they should be output from the buffer to the decoder within one second after they are input to the buffer. For this reason, a shift of the multiplexing of the video data and the audio data is one second at the maximum (strictly speaking, there is a further shift by the reorder of the video data).
While the video has been followed by the audio in this example, the audio can also be followed by the video in respect of theory. When a simple picture having a high compression ratio is prepared for the video data and the audio data are transferred unnecessary quickly, such data can be created intentionally. However, the precedence can be given within one second at the maximum based on the restrictions of the MPEG.
(Description of Tape Media)
Next, description will be given to tape media.
FIG. 40 is a diagram illustrating a relation between a tape and a tape recorder (recording head). As shown in the figure, recording areas for video data and audio data are separately provided in parallel with a tape running direction. Therefore it is easy to record audio data independently. In a conventional analog video tape recorder, reproduction and recording can be carried out simultaneously with one head since a delay time from reproduction to recording is almost zero.
The DVD-RAM expected as the next generation AV recording media for example has the following problems.
The biggest problem to an after-recording operation in the DVD recorder is caused by an MPEG stream which is AV data to be recorded by the DVD recorder and the difference of mechanism between a video recorder and the DVD recorder.
In the video recorder, as described in the prior art, each of video and audio channels is independently recorded on a tape. The after-recording operation of an audio can easily be carried out because there is no delay from playback to sound recording. In the DVD recorder, however, a video and an audio are recorded in one multiplexed stream. One optical pick-up is used for reading and writing operations. A time difference is made from playback to recording, since one track buffer is provided for implementing a variable bit rate. Even though two optical pick-ups are provided, they should be operated independently. Even if each of the optical pick-ups can be operated independently, the recording and playback operations cannot be carried out at the same time in the DVD-RAM for changing a rotating speed for each zone when a region to be accessed by each of the pick-ups is provided across different zones.
As described in the prior art, a time stamp for AV synchronous playback is described on the MPEG stream. Therefore, in the case where a time stamp to be given to an audio stream to be recorded later is inconsistent with a time stamp given to an existing stream a decoder is not normally operated in some cases. For example, in the case where an SCR given to a video pack in the existing stream and an SCR given to an audio pack recorded later have the same time, two data to be processed on the time of the SCR by the decoder are simultaneously present. Consequently, the decoder cannot be normally operated. Thus, there is a problem in that the worst hang up is caused.
The DVD-RAM can store audio streams in various kinds of format. The DVD recorder can not know whether the DVD recorder can apply an after-record operation to the DVD-RAM in which data has been recorded by other recorders. Therefore the stream must be analyzed at start of data recording.